The present invention relates to a switch for making and breaking an electric current and more particularly to to an arc-extinguishing mechanism thereof.
The present invention is applicable mainly to electromagnetic contactors and no-fuse breakers.
Referring first to FIG. 1, there is shown an example of a conventional electromagnetic contactor, having a symmetrical structure. In FIG. 1, the numeral 1 denotes a mounting base molded of a plastic material; the numeral 2 denotes a fixed iron core which comprises laminated silicon steel plates on the mounting base 1; the numeral 3 denotes a movable iron core which also comprises a lamination of silicon steel plates and is disposed in an opposed relation to the fixed iron core 2; the numeral 4 denotes an operating coil which provides a driving force for attracting the movable iron core 3 toward the fixed iron core 2 against a tripping spring (not shown); the numeral 5 denotes a cross bar formed of a plastic material, the cross bar 5 having a square window and holding the movable iron core 3 at its lower end. The numeral 6 denotes a movable contact plate inserted through the square window of the cross bar 5 and held in place by a pressure spring 7; the numeral 6A denotes a movable contact affixed to the movable contact plate 6; the numeral 8 denotes a fixed contact plate for making an electric circuit, the fixed contact plate 8 being disposed in an opposed relation to the movable contact plate 6; the numeral 8A denotes a fixed contact affixed to the fixed contact plate 8; and the numeral 8C denotes a terminal portion of the fixed contact plate 8.
Further, the numeral 9 denotes a terminal screw for connecting the electromagnetic contactor body to an external circuit; the numeral 10 denotes a base for mounting the fixed contact plate 8; and the numeral 11 denotes a cover which covers the upper portions of the electromagnetic contact plates. Inside the cover 11 is disposed a metallic arc deflecting plate 13 formed of a magnetic material for deflecting an arc 12 which is produced between the fixed contact 8A and movable contact 6A. FIG. 2 shows a mounting mode of the metallic arc deflecting plate 13 relative to the fixed contact plate 8 and fixed contact 8A. The metallic arc deflecting plate 13 is constituted as a channelshaped plate erected so as to surround the fixed contact 8A, and it is provided with a notched end 14 in a position close to the fixed contact plate 8 for insertion therethrough of the latter.
In this conventional electromagnetic contactor having the above-described construction, the movable iron core 3 moves away from the fixed iron core 2 by virtue of a tripping spring (not shown) when the operating coil 4 is deenergized. Since the cross bar 5 assumes the state shown in FIG. 1, at this time, the fixed and movable contacts 8A and 6A open producing the arc 12 between the fixed contact 8A and movable contact 6A. But the arc 12 is deflected by the metallic arc deflecting plate 13 and extinguished at the zero point of electric current, so that the electric current is cut off.
The extinguishing process for the arc 12 in such conventional electromagnetic contactor will now be described more in detail with reference to FIG. 3 which shows a section of an arc deflecting chamber. The arc 12 formed between the movable contact 6A and the fixed contact 8A is attracted by the metallic arc deflecting plate 13 of a magnetic material and stretched like an arc 12A. The resultant increase in arc voltage causes insulation breakdown between the fixed contact plate 8 and the metallic arc deflecting plate 13 and also between the notched end 14 of the metallic arc deflecting plate 13 and the movable contact 6A. As a result, the arc 12A is divided into two arcs 12B and 12C, which arcs are extinguished at the zero point of electric current. In the conventional electromagnetic contactor wherein the arc is extinguished according to such process, the stretch of arc is restricted by the presence of the metallic arc deflecting plate 13, and the arc voltage is low. As a result insulation breakdown is difficult to take place between the fixed contact plate 8 and the metallic arc deflecting plate 13 and also between the metallic arc deflecting plate 13 and the movable contact 6A. In other words, the arc 12 is difficult to be divided into two arcs 12B and 12C. Moreover, once the arc 12 is divided into two, the arc 12 tends to stay due to a field concentration at the end portion 14 which is inferior in thermal conductivity. Thus, not only remarkable breaking performance cannot be attained but it creates a major factor of degradation of the arc deflecting plate.